The present invention relates to a developing apparatus for an image forming apparatus, and especially to a developing apparatus which conducts development with a two-component developer, consisting of a magnetic carrier and a toner, and supplies as much toner as is consumed for development.
A developing apparatus in which a two-component developer is used for development, is generally composed of a developing sleeve, a stirring unit, and a toner container. The developer stirred in the stirring unit adheres to the circumferential surface of the developing sleeve and is conveyed to the developing region which faces a photoreceptor, in order to develop electrostatic latent images on the photoreceptor. After an amount of toner is consumed by the development, the developer adheres to the circumferential surface of the developing sleeve and is conveyed back to the stirring unit to circulate with the developer.
In order to realize good development when using a two-component developer, the toner content ratio in the developer (toner concentration) must be kept in a prescribed range. In order to detect the toner concentration, either magnetic detection of the developer in the stirring unit is conducted, or the standard latent image formed on the photoreceptor is developed and its reflection is measured to indirectly assess the toner concentration.
The method to detect the toner concentration by measuring the density of the developed standard latent image is indirect and does not necessarily indicate the exact toner concentration in the developer. The toner density measuring method by magnetic detection of the toner is problematic in that the results of the detection are not stable.
Even if as much toner is supplied to the developing unit as was consumed during development according to the measurement results, the toner supplying mechanism and its control system need to be complicated.
Developing methods in which two-component developer is used, are approximately classified into the following three methods.
(a) Two component developer magnetic brush developing method.
This is a conventional method. Brushlike bristles of developer are formed on a portion of the circumferential surface of the developing sleeve, facing the photoreceptor. The developer bristles come into contact with the photoreceptor on which a latent image has been formed. PA1 This developing method is disclosed in Japanese Patent Publication Open to Public Inspection No. 181362/1984 and No. 176069/1985. According to this developing method, a thin layer of developer is formed on the circumferential surface of the developing sleeve and development is conducted under the condition that there is a space between the toner on the sleeve and the photoreceptor. PA1 For example, according to Japanese Patent Publication Open to Public Inspection No. 42768/1985, only toner is adhered to the development sleeve and conveyed to the development region which faces the photoreceptor having a latent image on its surface to be developed.
(b) Two component developer non-contact developing method.
(c) One component developer non-contact developing method in which two component developer is used.
Concerning the toner concentration in the stirring unit, the density ranges of Method (b) and Method (c) are wider than the concentration ranges of Method (a). Furthermore, it has been found that excellent development can be carried out even if the concentration ranges are not maintained strictly. In the case of Method (a). the toner concentration range can be widened by using a carrier of a fine particle size.
The developing apparatus of the present invention has a feature that the toner concentration at the stirring unit can be controlled without using the conventional toner concentration detecting means and the toner supplying means based on the toner concentration detecting means. The developing apparatus of the invention is suitable for the above-mentioned developing methods (a). (b) and (c). and capable of supplying toner without using a complicated controlling system. The object of the present invention is to provide a developing apparatus with the features mentioned above.